In a gas turbine engine, ambient air is drawn into a compressor section. Alternate rows of stationary and rotating aerofoil blades are arranged around a common axis. Together these accelerate and compress the incoming air. A rotating shaft drives the rotating blades. Compressed air is delivered to a combustor section where it is mixed with fuel and ignited. Ignition causes rapid expansion of the fuel/air mix which is directed in part to propel a body carrying the engine and in another part to drive rotation of a series of turbines arranged downstream of the combustor. The turbines share rotor shafts in common with the rotating blades of the compressor and work, through the shaft, to drive rotation of the compressor blades.
The combustion process which takes place within the combustor of a gas turbine engine results in the walls of the combustor casing being exposed to extremely high temperatures. The alloys used in combustor wall construction are normally unable to withstand these temperatures without some form of cooling. It is known to take off a portion of the air output from the compressor (which is not subjected to ignition in the combustor and so is relatively cooler) and feed this to surfaces of the combustion chamber which are likely to suffer damage from excessive heat.
A casing enclosing the combustion chamber typically comprises a “dual-wall” structure wherein outer and inner wall elements are maintained in spaced apart relationship and cooling air is directed through holes in the outer wall into a channel defined between them. In addition, arrays of effusion holes are provided in the inner wall elements through which the cooling air is exhausted. The geometry and arrangement of the effusion holes is selected to provide a substantially continuous boundary layer of cooling air along the inner wall surface, protecting the component from the extremely hot combustion product generated in the combustion chamber.
For optimal effect, the arrays typically comprise groupings of 6-8 rows of effusion holes.
Interruptions to the boundary layer can arise where obstacles along the inner wall prevent the inclusion of a sufficiently proportioned array of effusion holes in a region of the inner wall. For example, the obstacle may be part of a fastener used to secure the inner and outer walls together, a dilution hole used for emissions control, or a join between the leading edge of a liner tile and the outer casing of a combustor. Such regions can be subjected to temperature profiles which impact on the mechanical properties of the wall over time and can result in a reduction in the operational life of the component.